If and Inline if, what are the advantages and disadvantages?

问题

I'm a little curious about the difference between if and inline if, in Python. Which one is better?

Is there any reason to use inline if, other than the fact that it's shorter?

Also, is there anything wrong with this statement? I'm getting a syntax error: SyntaxError: can't assign to conditional expression

a = a*2 if b == 2 else a = a/w

回答1:

The advantage of the inline if expression is that it's an expression, which means you can use it inside other expressions—list comprehensions, lambda functions, etc.

The disadvantage of the inline if expression is also that it's an expression, which means you can't use any statements inside of it.

A perfect example of the disadvantage is exactly what's causing your error: a = a/w is a statement, so you can't use it inside an expression. You have to write this:

if b == 2:
    a = a*2
else:
    a = a/w

Except that in this particular case, you just want to assign something to a in either case, so you can just write this:

a = a*2 if b==2 else a/w

As for the advantage, consider this:

odd_numbers = [number if number%2 else number+1 for number in numbers]

Without the if expression, you'd have to wrap the conditional in a named function—which is a good thing for non-trivial cases, but overly verbose here:

def oddify(number):
    if number%2:
        return number
    else:
        return number+1
odd_numbers = [oddify(number) for number in numbers]

Also, note that the following example is not using an if (ternary conditional) expression, but an if (conditional filter) clause:

odd_numbers = [number for number in numbers if number % 2]

回答2:

The correct way to use the conditional expression is:

result = X if C else Y

what you have is:

result = X if C else result = Y

So, you should remove the result = part from there. The major advantage of conditional expression is that, it's an expression. You can use them wherever you would use a normal expression, as RHS of assignment expression, as method/function arguments, in lambdas, in list comprehension, so on. However, you can't just put any arbitrary statements in them, like say print statements.

Fo e.g. suppose you want all even integers from a list, but for all odd numbers, you want the values as 0. You would use it in list comprehension like this:

result = [x if x % 2 == 0 else 0 for x in li]

回答3:

Inline if is an expression, so you can not put assignments inside.

Correct syntax would be:

a = a*2 if b == 2 else a/w

As for the usefulness, it's a question of style, and perhaps it would be a good question for Programmers StackExchange.

来源：https://stackoverflow.com/questions/19207722/if-and-inline-if-what-are-the-advantages-and-disadvantages

标签

python

if-statement

inline-if